This application is related to Japanese application No. 2000-298823 filed on Sep. 29, 2000, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a luminescent display device of active matrix drive type and a fabrication method therefor. More specifically, the invention relates to a luminescent display device of active matrix drive type employing thin film transistors and to a fabrication method therefor.
2. Description of the Related Art
In the recent advanced information age, flat panel display devices have increasingly been demanded.
Hitherto known as display devices of this type are non-luminescent display devices including liquid crystal displays (LCDs), and luminescent display devices including plasma display panels (PDPs) and electroluminescent displays (ELs) such as inorganic electroluminescent displays (IOELs) and organic electroluminescent displays (OELs). Among these display devices, the organic EL devices have shown a remarkable improvement.
A known technique for displaying a motion picture on such an organic EL display device is based on simple matrix driving (for example, Japanese Unexamined Patent Publication No. Hei 2(1990)-37385, and xe2x80x9cImportant Issues and Commercialization Strategy for Organic EL Devicexe2x80x9d p.55).
In the aforesaid driving method, scanning lines are sequentially driven. For driving a greater number of scanning lines (e.g., several hundreds scanning lines), an instantaneous luminance of several hundred thousands to several millions cd/m2 is required, and the driving method encounters the following problems.
(1) A higher driving voltage is required, so that interconnections suffer from a greater voltage drop.
(2) The driving is required in a lower luminous efficiency area in a higher luminance region, so that power consumption is increased.
To solve these problems, organic EL display devices have been developed which employ thin film transistors for active matrix driving (for example, Japanese Unexamined Patent Publications No. Hei 7(1995)-122360, No. Hei 7(1995)-122361, No. Hei 7(1995)-153576, No. Hei 8(1996)-241047 and No. Hei 8(1996)-227276, xe2x80x9cImportant Issues and Commercialization Strategy for Organic EL Devicexe2x80x9d p.62, IEDM98 p.875).
The organic EL display devices employing the active matrix driving method can be driven at a lower voltage. Since the driving in the higher luminous efficiency area is possible, the power consumption can significantly be reduced. Thus, the active matrix organic EL display devices are superior to the simple matrix organic EL display devices.
However, the organic EL display devices employ two or more thin film transistors for driving each pixel and, therefore, have a significantly reduced aperture ratio as compared with liquid crystal display devices (Japanese Unexamined Patent Publication No. Hei 7(1995)-111341, SID98 p.11).
To increase the pixel aperture ratio, as shown in FIGS. 27 to 29 or FIGS. 30 to 32, there has been proposed a display structure such that an organic EL device 8 including a pixel electrode 5, an EL layer 6 and a counter electrode 7 overlies a current control thin film transistor 212 or a switching thin film transistor 211 and a current control thin film transistor 212 with the intervention of an insulating film 12, and light emitted therefrom is outputted from an opposite side of a substrate 1 (for example, Japanese Unexamined Patent Publication No. 10-189252(1998)).
Where the electroluminescent device 8 which may be an inorganic EL device or an organic EL device is employed in combination with the thin film transistors as shown in FIGS. 27 to 29 and FIGS. 30 to 32, semiconductor layers 9 such as of amorphous silicon or polysilicon employed as active layers of the thin film transistors are liable to suffer from alteration or degradation in properties because of their photosensitivity when light emitted from the EL device 8 is incident on the semiconductor layers 9. This results in an increased leak current (off current). When a leak current in the switching thin film transistor is increased, charges accumulated in a capacitor are discharged through the switching thin film transistor. This makes it impossible to maintain light emission in a pixel for a proper time period, resulting in alteration in image quality. When a leak current in the current control thin film transistor is increased, a current continuously flows in the EL device, so that the EL device keeps emitting light even after the pixel is turned off, resulting in cross-talk. Further, the EL device consumes power even without light emission, resulting in an increased power consumption.
In view of the foregoing, the present invention is directed to a luminescent display device of active matrix drive type which is free from photo-degradation of active layers of thin film transistors and, hence, free from deterioration in characteristics of the thin film transistors thereby to have improved properties. The invention is further directed to a method for fabricating such a luminous display device.
In accordance with one aspect of the present invention, there is provided a luminescent display device of active matrix drive type, which comprises: a substrate; a switching thin film transistor, a current control thin film transistor, a capacitor, a signal line, a scanning line and a common line which are provided on the substrate; an electroluminescent device provided on the substrate and comprising a pixel electrode connected to the common line via the current control thin film transistor, an electroluminescent layer having at least one light emitting layer, and a counter electrode; and a light-shielding layer for preventing light emitted from the electroluminescent device from reaching the switching thin film transistor and the current control thin film transistor.
In accordance with another aspect of the present invention, there is provided a method for fabricating the aforesaid luminescent display device of active matrix drive type, wherein the at least one light emitting layer of the electroluminescent layer is formed by an evaporation method, a laser transfer method or a printing method.